Nitride based semiconductor light-emitting device

ABSTRACT

Disclosed herein is a nitride-based semiconductor light-emitting device. The nitride-based semiconductor light-emitting device comprises an n-type clad layer made of n-type Al x1 In y1 Ga (1-x1-y1) N (where 0≦x 1 ≦1, 0≦y 1 ≦1, and 0≦x 1 +y 1 ≦1), a multiple quantum well-structured active layer made of undoped In A Ga 1-A N (where 0&lt;A&lt;1) formed on the n-type clad layer, and a p-type clad layer formed on the active layer wherein the p-type clad layer includes at least a first layer made of p-type In y2 Ga 1-y2 N (where 0≦y 2 &lt;1) formed on the active layer and a second layer made of p-type Al x3 In y3 Ga (1-x3-y3) N (where 0&lt;x 3 ≦1, 0≦y 3 ≦1, and 0&lt;x 3 +y 3 ≦1) formed on the first layer.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/890,215, which is based on, and claims priority from, KoreanApplication Number 2004-10538, filed Feb. 18, 2004, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride-based semiconductorlight-emitting device, and more particularly to a nitride-basedsemiconductor light-emitting device which employs a p-type clad layerstructure with enhanced hole injection efficiency into an active layer,thereby exhibiting a high luminous efficiency.

2. Description of the Related Art

Generally, nitride-based semiconductor light-emitting devices areoptical devices with a high output that generate short-wavelength lightin the blue and green ranges and the like, and thus enable realizationof the full color spectrum. For these reasons, nitride-basedsemiconductor light-emitting devices have drawn attention in relatedindustrial fields.

Nitride-based semiconductor light-emitting devices are semiconductorsingle crystals composed of Al_(x)In_(y)Ga_((1-x-y))N (wherein 0≦x≦y,0≦y≦1 and 0≦x+y≦1) which can be grown on substrates, e.g., sapphire andSiC substrates, by metal organic chemical vapor deposition (MOCVD).

Nitride-based semiconductor light-emitting devices essentially consistof an n-type clad layer, an undoped active layer and a p-type cladlayer. A conventional nitride-based semiconductor light-emitting devicesis sectionally shown in FIG. 1.

Referring to FIG. 1, the conventional nitride-based semiconductorlight-emitting device 10 comprises a sapphire substrate 11, and ann-type clad layer 13, an undoped active layer 15 and a p-type clad layer17 deposited in this order on the sapphire substrate 11. In addition,the light-emitting device 10 further comprises an n-side electrode 19 aand a p-side electrode 19 b which are connected to the n-type clad layer13 and the p-type clad layer 17, respectively. The active layer 15 mayhave a multiple quantum well structure in which a plurality of GaNquantum barrier layers and a plurality of InGaN quantum well layers arealternately laminated.

When an electric current is applied to the electrodes 19 a and 19 b,electrons emitted from the n-type clad layer 13 and holes generated fromthe p-type clad layer 17 are recombined together in the active layer 15having a multiple quantum well structure to emit short-wavelength lightin the green or blue ranges.

As generally illustrated in FIG. 1, the p-type clad layer 17 includes anelectron blocking layer (EBL) 17 b formed on the active layer 15 and acontact layer 17 a formed on the electron blocking layer 17 b. Theelectron blocking layer 17 b may be made of a nitride semiconductorcontaining Al, such as p-type AlGaN, whereas the contact layer 17 a maybe made of a nitride semiconductor containing no Al, such as GaN.

Since the AlGaN electron blocking layer 15 b has a larger energy bandgap than a nitride semiconductor containing no or a small amount of Al,it can effectively prevent overflow of electrons emitted from the n-typeclad layer 13 without recombination with holes in the active layer. Assuch, the p-type clad layer 17 including the EBL 17 b can decrease thenumber of overflowing electrons, contributing to an improvement in theluminous efficiency of the light-emitting device 10.

However, since AlGaN not only has a lower hole mobility than any othernitride semiconductor layers, but also has a relatively low holeconcentration (about 1×10¹⁷/cm³), the injection efficiency of holesgenerated from the p-side electrode 19 b to the active layer 15 may belowered, thus causing a problem in obtaining a high luminous efficiency.

Thus, there is a need in the art for a novel nitride-basedlight-emitting device which maintains the advantage of an EBL, i.e.,prevention of electron overflow, and at the same time, enhances theefficiency of holes injected into an active layer, thereby remarkablyimproving the overall luminous efficiency in a complementary manner.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide anitride-based semiconductor light-emitting device which employs a p-typeclad layer structure consisting of an electron blocking layer (EBL), anactive layer and a layer for enhancing hole injection efficiency(hereinafter referred to as a “hole injection efficiency-enhancinglayer”) interposed therebetween, thereby preventing electrons fromoverflowing and enhancing hole injection efficiency.

In order to accomplish the above object of the present invention, thereis provided a nitride-based semiconductor light-emitting devicecomprising an n-type clad layer made of n-typeAl_(x1)In_(y1)Ga_((1-x1-y1))N (where 0≦x₁≦1, 0≦y₁≦1, and 0≦x₁+y₁≦1), amultiple quantum well-structured active layer made of undopedIn_(A)Ga_(1-A)N (where 0<A<1) formed on the n-type clad layer, and ap-type clad layer formed on the active layer wherein the p-type cladlayer includes at least a first layer made of p-type In_(y2)Ga_(1-y2)N(where 0≦y₂ <1) formed on the active layer and a second layer made ofp-type Al_(x3)In_(y3)Ga_((1-x3-y3))N (where 0<x₃≦1, 0≦y₃≦1, and0<x₃+y₃≦1) formed on the first layer.

Preferably, the first layer is formed in such a manner that it has ahigher doping concentration than the second layer. The thickness of thefirst layer is preferably adjusted to about 100 nm or less, and morepreferably within the range of about 10 nm to about 30 nm.

In one embodiment of the present invention, the p-type clad layer mayfurther include a third layer, as a p-type contact layer, made of p-typeIn_(y4)Ga_(1-y4)N (where 0≦y₄<1) formed on the second layer.

The third layer may consist of a low concentration p-type GaNsemiconductor layer having a first doping concentration formed on thesecond layer, and a high concentration p-type GaN semiconductor layerhaving a concentration higher than the first doping concentration formedon the low concentration p-type GaN semiconductor layer.

In this embodiment, the thickness of the second layer is preferablyadjusted within the range of about 50 nm to about 200 nm so as tosufficiently prevent overflow of electrons. In this case, the totalthickness of the p-type clad layer may be at least 160 nm.

Preferably, the first layer is a GaN semiconductor layer, and the secondlayer is an AlGaN semiconductor layer. Further, the n-type clad layer ispreferably a GaN semiconductor layer.

In order to enhance the injection efficiency of holes generated from thep-type clad layer while preventing current loss due to electronoverflow, the p-type clad layer includes an electron blocking layer(EBL) as a nitride semiconductor layer containing Al, an active layer asa nitride semiconductor layer containing Al, and a nitride semiconductorlayer containing no Al interposed between the two layers. The nitridesemiconductor layer containing no Al, such as GaN, has a higher holemobility (about 15 cm²/Vs to about 20 cm²/Vs) than the EBL containing Al(about 5 cm²/Vs to about 10 cm²/Vs). Further, from the viewpoint of holeconcentration, a p-type GaN layer has a higher hole concentration(5×10¹⁷/cm³) than a p-type AlGaN layer (about 1×10¹⁷/cm³).

In conclusion, since the nitride semiconductor layer containing no Al,such as GaN, is interposed between the EBL and the active layer,electron overflow is prevented by the EBL containing Al. At the sametime, since the nitride semiconductor layer containing no Al is placedin contact with the active layer, hole injection efficiency is markedlyenhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a side cross-sectional view of a conventional nitride-basedsemiconductor light-emitting device;

FIG. 2 is a side cross-sectional view of a nitride-based semiconductorlight-emitting device according to one embodiment of the presentinvention;

FIG. 3 is a side cross-sectional view of a nitride-based semiconductorlight-emitting device according to another embodiment of the presentinvention; and

FIG. 4 is a graph comparing the luminance characteristics between anitride-based semiconductor light-emitting device fabricated inComparative Example 1 and nitride-based semiconductor light-emittingdevices fabricated in Examples 1 and 2 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed description will be made of embodiments of the presentinvention with reference to the accompanying drawings.

FIG. 2 is a side cross-sectional view of a nitride-based semiconductorlight-emitting device according to one embodiment of the presentinvention.

As illustrated in FIG. 2, the nitride-based semiconductor light-emittingdevice 20 according to one embodiment of the present invention comprisesa sapphire substrate 21, and an n-type clad layer 23, an undoped activelayer 25 and a p-type clad layer 27 deposited in this order on thesapphire substrate 21. The light-emitting device 20 further comprises ann-side electrode 29 a and a p-side electrode 29 b which are connected tothe n-type clad layer 23 and the p-type clad layer 27, respectively. Theactive layer 25 may have a multiple quantum well structure in which aplurality of GaN quantum barrier layers and a plurality of InGaN quantumwell layers are alternately laminated.

The n-type clad layer 23 formed on the sapphire substrate 21 may be aGaN layer, but is not limited thereto. As an example, the n-type cladlayer 23 may be made of n-type Al_(x)—In_(y1)Ga_((1-x1-y1))N (where0≦x₁≦1, 0≦y₁≦1, and 0≦x₁ +y ₁≦1).

The active layer 25 formed on the n-type clad layer 23 may have amultiple quantum well structure, for example, a structure wherein aplurality of InGaN-based quantum well layers and a plurality ofGaN-based quantum barrier layers are alternately laminated.

In this embodiment, the p-type clad layer 27 includes a first layer 27 cfor enhancing the hole injection efficiency, a second layer 27 b as anEBL (electron blocking layer), and if necessary, a third layer 27 a as acontact layer. The first layer 27 c is formed on the active layer 25 andmade of p-type In_(y2)Ga_(1-y2)N (where 0≦y₂<1), such as GaN. The secondlayer 27 b is formed on the first layer 27 c and made of p-typeAl_(x3)In_(y3)Ga_((1-x3-y3))N (where 0<x₃≦1, 0≦y₃<1 and 0<x₃+y₃≦1). Thethird layer 27 a may be made of p-type In_(y4)Ga_(1-y4)N (where 0≦y₄<1).

As such, the first layer 27 a, as a p-type contact layer, is made of anitride semiconductor containing no Al, such as p-type GaN. The secondlayer 27 b, as an electron blocking layer, is made of a nitridesemiconductor containing Al, such as p-type AlGaN. The first layer 27 cis made of a nitride semiconductor containing Al, such as p-type GaN.Since the first layer 27 c containing no Al has a higher hole mobilityand a higher impurity concentration than the second layer 27 b, as anEBL, containing Al, it is advantageous in enhancing the hole injectionefficiency.

For example, p-type AlGaN constituting a common p-type EBL has a holemobility of about 5˜10 cm²/Vs and an impurity concentration of about1×10¹⁷/cm³. In contrast, the p-type GaN layer that is employed as thelayer for enhancing the hole injection efficiency in the presentinvention has a hole mobility of about 15 cm²/Vs to about 20 cm²/Vs andan impurity concentration of about 5×10¹⁷/cm³.

Accordingly, the hole injection efficiency-enhancing layer 27 ccontaining no Al, such as GaN, interposed between the EBL 27 b and theactive layer 25 prevents electrons from overflowing due to the presenceof the EBL 27 b containing Al, and at the same time, markedly enhancesthe hole injection efficiency because it is formed on the active layer25.

The hole injection efficiency-enhancing layer 27 c as the third layer ispreferably formed in such a manner that it has a higher p-type impurity(e.g., Mg) concentration than the EBL 27 b as the second layer in orderto further increase the hole injection efficiency. The larger thethickness of the hole injection efficiency-enhancing layer 27 c, thegreater the distance between the EBL 27 b and the active layer 25. Forthis reason, it is difficult to achieve the function of the EBL 27 b,i.e., prevention of electron overflow. Accordingly, the thickness of thehole injection efficiency-enhancing layer 27 c is preferably adjusted toabout 100 nm or less.

According to another aspect of the present invention, since thestructure of the light-emitting device can greatly enhance the holeinjection efficiency by using the hole injection efficiency-enhancinglayer, such as GaN layer, interposed between the EBL and the activelayer, it preferably permits the EBL to have a large thickness in orderto efficiently prevent electron overflow without deteriorating the holeinjection efficiency. The two functions, that is, hole injectionefficiency and prevention of electron overflow, have been recognized, inconventional light-emitting devices, as working based on mutuallyexclusive principles. However, in the present invention, since theintroduction of the hole injection efficiency-enhancing layer basicallyremoves negative results (e.g., reduction of the hole injectionefficiency) due to increased thickness of the EBL, the two functions canbe improved in a complementary manner.

FIG. 3 is a side cross-sectional view of a nitride-based semiconductorlight-emitting device according to another embodiment of the presentinvention.

The nitride-based semiconductor light-emitting device 30 illustrated inFIG. 3 comprises a sapphire substrate 31, and an n-type clad layer 33,an undoped active layer 35 and a p-type clad layer 37 deposited in thisorder on the sapphire substrate 31. The light-emitting device 30 furthercomprises an n-side electrode 39 a and a p-side electrode 39 b which areconnected to the n-type clad layer 33 and the p-type clad layer 37,respectively. Similarly to the structure shown in FIG. 2, the p-typeclad layer 37 includes a first layer 37 c for enhancing the holeinjection efficiency, a second layer 37 b as an EBL, and if necessary, athird layer 37 a as a contact layer.

The first layer 37 c for enhancing the hole injection efficiency may bemade of p-type GaN, and the second layer 37 b as an EBL may be made ofp-type AlGaN. Further, the third layer 37 a as a contact layer of thep-type clad layer may be made of p-type GaN. The third layer 37 apreferably includes a first p-type GaN layer having a first impurityconcentration and a second p-type GaN layer having a second impurityconcentration higher than the first impurity concentration.

As explained previously, the thickness (t₁) of the first layer 37 c ispreferably adjusted to about 100 nm or less, and more preferably withinthe range of about 10 nm to about 30 nm, so as not to deteriorate thefunction of the EBL. Above about 10 nm, sufficient hole injectionefficiency can be achieved. Below about 30 nm, there is no deteriorationin the function of the EBL.

In addition, to sufficiently ensure the function of the EBL, thethickness (t₂) of the second layer 37 b is preferably in the range ofabout 50 nm to about 200 nm. Conventionally, based on problemsassociated with crystal growth of nitride semiconductors containing Aland reduction in hole injection efficiency due to improved function ofEBL, the thickness of the EBL has been adjusted to less than 50 nm. Incontrast, since the hole injection efficiency-enhancing layer is incontact with the active layer in the present invention, the holeinjection efficiency is enhanced while effectively preventing electronoverflow.

Under these circumstances, taking account of the thickness of the thirdlayer 37 a as a contact layer, the p-type clad layer 37 employed in thepresent invention is preferably formed to a thickness of at least 160nm.

Hereinafter, the operation and effects of the present invention will beexplained in more detail with reference to specific examples.

EXAMPLE 1

In order to identify the improvement in the characteristics of thenitride-based semiconductor light-emitting device according to thepresent invention, the nitride-based semiconductor light-emitting deviceshown in FIG. 2 was fabricated.

First, after a GaN low temperature-nuclei growing layer as a bufferlayer was formed on a sapphire substrate, an n-type GaN clad layer wasformed thereon so as to have an impurity concentration of 4×10¹⁸/cm³.Thereafter, a multiple quantum well-structured active layer composed offive In_(0.15)Ga_(0.85)N quantum well layers and five GaN quantumbarrier layers was formed on the n-type clad layer.

Further, a hole injection efficiency-enhancing layer made of p-type GaN,an electron blocking layer (EBL) made of p-type Al_(0.15)Ga_(0.85)N, anda contact layer made of p-type GaN were deposited sequentially on theactive layer to prepare a p-type clad layer. The hole injectionefficiency-enhancing layer was formed so as to have an impurityconcentration of about 5×10¹⁷/cm³ and a thickness of about 20 nm. Theelectron blocking layer was formed so as to have an impurityconcentration of about 1×10¹⁷/cm³ and a thickness of about 20 nm.Further, the contact layer was formed so as to have an impurityconcentration of about 5×10¹⁷/cm³ and a thickness of about 76 nm.

Finally, a transparent electrode layer containing Ni was formed on thep-type clad layer, and then an n-side electrode and a p-side electrodewere formed on the n-type clad layer and the p-type clad layer,respectively, to fabricate the final nitride-based semiconductorlight-emitting device.

EXAMPLE 2

To observe changes in the characteristics of the nitride-basedsemiconductor light-emitting device of the present invention accordingto increasing thickness of the p-type EBL, the thickness of the EBL wasincreased, compared to in the light-emitting device fabricated inExample 1.

In this example, a nitride-based semiconductor light-emitting device wasfabricated in the same manner as in Example 1, except that the structureof the p-type clad layer was varied. Specifically, a hole injectionefficiency-enhancing layer was made of p-type GaN so as to have animpurity concentration of about 5×10¹⁷/cm³ and a thickness of about 20nm, an electron blocking layer was made of p-type Al_(0.15)Ga_(0.85)N soas to have an impurity concentration of 1×10¹⁷/cm³ and a thickness ofabout 69 nm, and a p-type contact layer was made of p-type GaN so as tohave an impurity concentration of 5×10¹⁷/cm³ and a thickness of about 76nm.

COMPARATIVE EXAMPLE 1

A nitride-based semiconductor light-emitting device was fabricated inthe same manner as in Example 1, except that the hole injectionefficiency-enhancing layer was not formed so that the p-type clad layerincludes the EBL and the contact layer only. Specifically, the EBL andthe contact layer were sequentially formed on the active layer withoutthe formation of the hole injection efficiency-enhancing layer. Theelectron blocking layer made of p-type Al_(0.15)Ga_(0.85)N was formed soas to have an impurity concentration of about 1×10¹⁷/cm³ and a thicknessof about 20 nm, and the p-type contact layer made of p-type GaN wasformed so as to have an impurity concentration of about 5×10¹⁷/cm³ and athickness of about 76 nm, as in Example 1.

To observe the electrical properties of the nitride-based semiconductorlight-emitting devices fabricated in Examples 1 and 2, and ComparativeExample 1, first, forward voltage characteristics (at 20 mA) of thenitride-based semiconductor light-emitting devices of Example 1 andComparative Example 1 were measured. As a result, the nitride-basedsemiconductor light-emitting device of Comparative Example 1 was shownto have a forward voltage of about 3.39V, whereas the nitride-basedsemiconductor light-emitting device of Example 1 into which the holeinjection efficiency-enhancing layer made of p-type GaN was introducedwas shown to have a forward voltage of 3.09V. That is, it was confirmedthat the presence of the hole injection efficiency-enhancing layer inthe nitride-based semiconductor light-emitting device of the presentinvention lowered the forward voltage by about 0.3V, indicating that theforward voltage characteristics were improved.

Similarly to this, the reverse voltage characteristics of thenitride-based semiconductor light-emitting devices fabricated inExamples 1 and 2 (at −20 mA) were measured. The nitride-basedsemiconductor light-emitting device of Example 1 was shown to have areverse voltage of about 9.5V, whereas the nitride-based semiconductorlight-emitting device of Example 2 was shown to have a reverse voltageof about 10.6V. It was confirmed from these results that thenitride-based semiconductor light-emitting device of Example 2 had ahigher reverse voltage by about 0.9V than the nitride-basedsemiconductor light-emitting device of Example 1.

It has been demonstrated that the introduction of the hole injectionefficiency-enhancing layer and increased thickness of the p-type EBL canenhance the hole injection efficiency and effectively prevent electronoverflow.

Additionally, the luminance characteristics of the nitride-basedsemiconductor light-emitting devices of Examples 1 and 2, andComparative Example 1 were observed. The luminance characteristics weremeasured in the wavelength region of 464 nm under the same conditions.The results are shown in FIG. 4.

The nitride-based semiconductor light-emitting device of ComparativeExample 1 had a luminance of only 10.9 (Arbitrary unit), whereas thenitride-based semiconductor light-emitting device of Example 1 whereinthe p-type GaN hole injection efficiency-enhancing layer was interposedbetween the EBL and the active layer had a luminance of about 13.0 (Arb.unit), which was higher by about 2.1. In addition, the nitride-basedsemiconductor light-emitting device of Example 2 wherein the holeinjection efficiency-enhancing layer was introduced and the thickness ofthe p-type EBL was as large as 6.9 nm had a luminance of about 13.3(Arb. unit), which was higher by about 0.3 than that of Example 1.

In conclusion, since the hole injection efficiency-enhancing layercontaining no Al, such as GaN, as a nitride semiconductor layer, isinterposed between the EBL and the active layer, and at the same time,is placed in contact with the active layer, the hole injectionefficiency can be markedly enhanced. Moreover, since the EBL withincreased thickness effectively prevents electrons from overflowing, theelectrical properties and luminance characteristics can be improved.

The scope of the present invention is not limited by the aboveembodiments and the accompanying drawings, but only by the appendedclaims. Therefore, those skilled in the art will appreciate that variousmodifications, additions and substitutions are possible, withoutdeparting from the technical spirit of the invention as disclosed in theappended claims.

As apparent from the above description, since the nitride-basedsemiconductor light-emitting device of the present invention employs ap-type clad layer in which a nitride semiconductor layer is interposedbetween an EBL and an active layer, the function of the EBL, i.e.,prevention of electron overflow, is maintained, and at the same time,the efficiency of holes injected into an active layer is remarkablyenhanced. In addition, since the nitride-based semiconductorlight-emitting device of the present invention permits the use of an EBLhaving a large thickness in order to efficiently prevent electronoverflow without deteriorating the hole injection efficiency, it canimprove the two functions, that is, hole injection efficiency andprevention of electron overflow, based on mutually exclusive principles,in a complementary manner.

1. A nitride-based semiconductor light-emitting device, comprising: ann-type clad layer made of n-type Al_(x1)In_(y1)Ga_((1-x1-y1))N (where0≦x₁≦1, 0≦y₁≦1, and 0≦x₁+y₁≦1); a multiple quantum well-structuredactive layer made of undoped In_(A)Ga_(1-A)N (where 0<A<1) formed on then-type clad layer; and a p-type clad layer formed on the active layer,including at least a first layer made of p-type In_(y2)Ga_(1-y2)N (where0≦y₂≦1) formed on the active layer and a second layer made of p-typeAl_(x3)In_(y3)Ga_((1-x3-y3))N (where 0≦x₃≦1, 0≦y₃≦1, and 0<c₃+y₃≦1)formed on the first layer; wherein the first layer has a higher dopingconcentration than the second layer; and wherein the first layer has athickness of about 10 nm to about 30 nm. 2-4. (canceled)
 5. Thenitride-based semiconductor light-emitting device according to claim 1,wherein the p-type clad layer further includes a third layer made ofp-type In_(y4)Ga_(1-y4)N (where 0≦y₄<1) formed on the second layer. 6.The nitride-based semiconductor light-emitting device according to claim5, wherein the third layer consist of a low concentration p-type GaNsemiconductor layer having a first doping concentration formed on thesecond layer, and a high concentration p-type GaN semiconductor layerhaving a concentration higher than the first doping concentration formedon the low concentration p-type GaN semiconductor layer.
 7. Thenitride-based semiconductor light-emitting device according to claim 5,wherein the second layer has a thickness of about 50 nm to about 200 nm.8. The nitride-based semiconductor light-emitting device according toclaim 7, wherein the p-type clad layer has a thickness of at least 160nm.
 9. The nitride-based semiconductor light-emitting device accordingto claim 1, wherein the first layer is a GaN semiconductor layer and thesecond layer is an AlGaN semiconductor layer.
 10. The nitride-basedsemiconductor light-emitting device according to claim 1, wherein then-type clad layer is a GaN semiconductor layer.